Group III nitride semiconductor light emitting diodes offer a direct transition over a band gap energy from visible light to ultraviolet rays, and excel in the light emission efficiency, and thus have been used as light-emitting devices such as LED and LD.
In addition, when used for an electronic device, Group III nitride semiconductors have a potential to provide electronic devices having characteristics superior to those using conventional Group III-V compound semiconductors.
Such Group III nitride compound semiconductors are, in general, produced from trimethyl gallium, trimethyl aluminum, and ammonia as raw materials through a Metal Organic Chemical Vapor Deposition (MOCVD) method. The MOCVD method is a method in which a vapor of a raw material is introduced into a carrier gas to convey the vapor to the surface of a substrate and decompose the raw material by the reaction with the heated substrate, to thereby grow a crystal.
Hitherto, a single crystal wafer of a Group III-V compound semiconductor is, in general, produced by growing a crystal on a single crystal wafer of a different material. There is a large lattice mismatching between such a different kind of substrate and a Group III nitride semiconductor crystal to be epitaxially grown thereon. For example, when gallium nitride (GaN) is grown on a sapphire (Al2O3) substrate, there is a lattice mismatching of 16% therebetween, and when gallium nitride is grown on a SiC substrate, there is a lattice mismatching of 6% therebetween. In general, a large lattice mismatching as in the above leads to a problem in that it is difficult to epitaxially grow a crystal directly on a substrate, or a crystal, even if grown, can not gain excellent crystallinity.
Thus, for epitaxially growing a Group III nitride semiconductor crystal on a single crystal sapphire substrate or a single crystal SiC substrate through a Metal Organic Chemical Vapor Deposition (MOCVD) method, a method has been proposed and generally performed in which, firstly, a layer called a low temperature buffer layer made of aluminum nitride (AlN) or aluminum nitride gallium (AlGaN) is laminated on a substrate, and then a Group III nitride semiconductor crystal is epitaxially grown thereon at a high temperature (for example, Patent Documents 1 and 2).
In addition, there has been proposed a method for forming a film of AlN on a substrate by the system called face-to-face cathodes in which targets are positioned face-to-face (for example, Non-Patent Document 1).
In addition, there has been proposed a method for forming a film of AlN on a substrate by a DC magnetron sputtering method (for example, Non-Patent Document 2).
In addition, regarding a method in which a layer of such as AlN is formed as a barrier layer by a method other than the MOCVD method, and another layer is formed thereon by a MOCVD method, a method has been proposed in which a buffer layer is formed by high frequency sputtering, and a crystal having the same composition is grown thereon by a MOCVD method (for example, Patent Document 3). However, the method disclosed in Patent Document 3 has a problem in that an excellent crystal cannot be stably produced.
Thus, in order to stably produce an excellent crystal, for example, there have been proposed a method for annealing a buffer layer in a mixed gas made of ammonia and hydrogen on completion of its growth (for example, Patent Document 4), and a method for forming a buffer layer by DC sputtering at a temperature of 400° C. or higher (for example, Patent document 5). In the methods described in Patent Documents 4 and 5, a material such as sapphire, silicon, silicon carbide, zinc oxide, gallium phosphide, gallium arsenide, magnesium oxide, manganese oxide, or a Group III nitride type compound semiconductor single crystal is used as a substrate material. Of these, the substrate of the a surface of sapphire is considered to be preferable.
However, a buffer layer is formed by a sputtering method in the methods described in Patent Documents 4 and 5. Therefore, while the film formation rate is high, a buffer layer with poor crystallinity can be formed depending on film formation conditions. When a GaN layer is grown on such buffer layer with poor crystallinity, the crystallinity of the GaN layer is significantly deteriorated.
On the other hand, research has been conducted on the manufacture of a Group III nitride compound semiconductor crystal by sputtering. For example, there has been proposed a method for forming a Ga layer on the (100) surface of Si and the (0001) surface of Al2O3 by a high-frequency magnetron sputtering using N2 gas has been proposed (for example, Non-Patent Document 3).
In addition, there has been proposed a method for forming a GaN layer on a substrate using an apparatus in which a cathode and a solid target are positioned face-to-face and mesh is provided between a substrate and a target (for example, Non-Patent Document 4).
In addition, there has been proposed a method for forming a GaN layer in which the energy of the provided particles, which are withdrawn from a target and collides to a substrate, is reduced to such a low energy as possible (for example, Non-Patent Document 5).    Patent Document 1: Japanese Patent No. 3026087    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. Hei 4-297023    Patent Document 3: Japanese Examined Patent Application, Second Publication No. Hei 5-86646    Patent Document 4: Japanese Patent No. 3440873    Patent Document 5: Japanese Patent No. 3700492    Non-Patent Document 1: Kikuo Tominaga, et al., “Japanese Journal of Applied Physics”, Vol. 28, p 7 (1989)    Non-Patent Document 2: M. Ishihara, et al., “Thin Solid Films”, Vol. 316, p 152 (1998)    Non-Patent Document 3: Y. USHIKU, et al. “21 Century Union Symposium Collected Papers”, Vol. 2nd, p 295 (2003)    Non-Patent Document 4: T. Kikuma, et al., “Vacuum”, Vol. 66, p 233 (2002)    Non-Patent Document 5: Kikuo Tominaga, et al., “Journal of Vacuum Science and Technology”, A22, p 1587 (2004)
In the methods described in Non-Patent Documents 3 and 4, when a GaN layer is formed by a sputtering method, a target is cooled by means of water-cooling, etc. so as to be used as a solid target. This is because Ga used as a target is a metal having a low melting point of about 29° C. However, when a GaN layer is formed by a sputtering method using such solid target, the continuous application of a power to a target causes the partially disproportionate reduction of the target. Therefore, there is a problem in that the film formation rate of the GaN layer varies as a function of the target amount.
Moreover, in the method described in Non-Patent document 5, during the formation of a GaN layer by a sputtering method, the sputtering is performed while the energy of the provided particles, which are withdrawn from a target and collides to a substrate, is reduced. Therefore, there is a problem in that a GaN layer with good crystallinity cannot be formed.
Accordingly, when a GaN layer with good crystallinity is stably formed on a substrate by a sputtering method, the particles frying out of the target during the sputtering is desired to be particles with much high energy.